Many weighing systems are known, some dating back to biblical times. More recently, weighing systems have been developed for weighing each one of a stream of articles, such as mail pieces or parcels moving through a transport or mail sorting system. Prior art systems of that type are shown, for example, in U.S. Pat. Nos. 7,096,152 and 3,648,839.
Some known systems rely on back-EMF or “Electro Magnetic Force Restoration” principles. According to one vendor, “an applied load is compensated for by an electromagnetically produced counterforce. A precision position control (optical) keeps the system stable. The slightest movement is detected, initiates a feedback circuit to run current through a coil and causes the load to be returned to its original position. The coil current, which is proportional to the weight, is transmitted to an internal A/D converter then processed in the microprocessor.”
The need remains for improvements in accuracy, speed (throughput), reduced cost, and reducing the need for frequent re-calibration of weighing systems. The present application achieves these and other goals by applying a dramatically different approach to the problem as further explained below.
Postal services typically charge for delivery of mail pieces by their weight, among other criteria. In general, the heavier the mail piece, the more is charged. Bulk mailers, who may mail thousands of pieces at a time, sometimes intermix heavier mail pieces with lighter ones, and put the postage appropriate for the lighter pieces on every piece. This can occur, for example, when a bank mails out its customer statements. Most statements contain a few sheets and easily fit under the one ounce cut-off, but some of them contain many sheets and are overweight. Very often the banks do not put proper postage on the heavier pieces.
Audits, sometimes run by manually weighing suspect mail pieces, indicate that the US Postal Service loses many millions of dollars each year because of this practice. Because current methods of weighing mail pieces are either too slow for existing sorting machines or require individual or a set of mail pieces to be weighed manually, postal services have not devised methods for ensuring proper postage on each mail piece. It is important to remember that postage applies to each mail piece, not to the average weight within a set of mail pieces. As a result, a set of mail pieces whose average weight is under the limit may still contain many mail pieces that are individually too heavy and require greater postage.